This invention relates generally to a novel method and apparatus for preventing undesirable conditions in electric discharge machining using pulses of electric discharge energy across a gap between an electric discharge machining electrode and a workpiece. More particularly, the invention concerns detecting the absence or low levels of radio frequency energy across the gap during electric discharge machining and causing the electrode to rapidly withdraw from the workpiece in response to the detection of the absence or low levels of the radio frequency energy, which absence or low levels indicates the presence or onset of undesirable damaging conditions in the electric discharge machining.
During electrical discharge machining (EDM) it is desirable to prevent arcing or short circuit conditions. Arcing can occur, e.g., when flushing with the bath of, e.g., oil, between the workpiece and EDM electrode is inadequate or non-existent, when there is an improper distance between the machining electrode and the workpiece or when the EDM pulse cutting current levels are too high. Short circuit conditions can occur due to improper flushing resulting in removed pieces of the workpiece not being flushed and lodging between the workpiece and EDM electrode, causing a short circuit or by the electrode touching the workpiece.
Arcing typically occurs when using Graphite or Copper Graphite EDM electrodes. The arcing is similar to that which occurs in a carbon arc lamp. Many times it is not practical to always maintain perfect flushing conditions due to electrode dimensions or workpiece shape.
Arcing conditions, depending on the severity, can severely damage the workpiece or the electrode, or both. Further, a sustained arcing condition can result in severe damage to the EDM machine and destruction of the shop through fire in the EDM system.
In EDM systems a servo-mechanism is typically used to control the distance between the EDM electrode and the surface of the workpiece being machined in order to optimize the machining conditions. The servo-mechanism for insuring proper sparking between the workpiece and electrode is, in reality, typically a voltage regulator. The average voltage of the EDM machining pulses across the EDM electrode is compared to a reference voltage, and an error signal generated in response to this comparison and used to control the position of the electrode with respect to the workpiece.
In EDM systems currently in use, error signals (UP signals) indicating the electrode is too close to the workpiece typically are on the order of one-third to one-fourth the magnitude or error signals indicating the electrode (DOWN signals) is too far from the workpiece. Thus the servo control mechanism, which is responsive to the polarity and magnitude of the error signal, will cause the electrode to be inserted towards the workpiece at a rate faster than it will cause the electrode to withdraw from the workpiece. This is typically the case when the EDM machine is set for a high efficiency cut.
This condition of higher insertion velocity than withdrawal velocity can cause instability in the EDM system, or cause the electrode to withdraw too slowly or be reinserted too quickly during undesirable short circuit or arcing condition.
Any suitable servo-mechanism can be used, and many are used in the art, to control the gap distance between the electrode and the workpiece in response to the error signal.
Prior art systems attempted to measure voltage across the gap in order to detect abnormal conditions and withdraw the electrode to prevent unwanted arcing and short circuits. However, when arcing or a low resistance path due to unflushed contamination occurs, the average voltage across the gap can be of the same magnitude as when the proper condition of sparking across the gap occurs. Thus the servo-control system would not be signaled to retract the electrode, remaining in that position resulting in arc damage to the electrode or workpiece or both.
The frequency generated by a sparking condition across a gap between an electrode and workpiece has been used in electro-chemical machining (ECM) to detect the existence of undesired sparking during ECM and withdraw the electrode, as shown in the patent to Dehner, U.S. Pat. No. 3,652,440 of Mar. 28, 1972. However, sparking is the desired mode of operation in EDM operations wherein the spark discharges cause the machining of the workpiece to occur.
In EDM systems the electrical energy supplied to the gap to perform the machining is in the form of square wave pulses which can be positive or negative depending on the desired cutting polarity. Typically EDM systems have a pulse repetition rate of from about 1 KHZ to about 250 KHZ, a pulse width of from 3 microseconds to 1 millisecond. Peak machining currents are also typically varied from 0.5 amps to 60 amps, however, power supplies are available to supply up to about 1000 amps. Open circuit voltages with the electrodes removed from the working vicinity of the workpiece can vary typically from 60 to 300 volts.
When the open circuit voltage of the EDM machine is impressed across the gap between the electrode and workpiece with the electrode in the normal desired work position, sufficiently close to allow proper sparking, a wave form of voltage is created across the gap with an initial relatively large spike followed by a voltage at some level above zero. This large spike is caused by the fact that it takes a finite time, on the order of 1 microsecond, to ionize the EDM fluid bath, typically oil, between the electrode and the workpiece.
This wave form is present during all conditions of EDM pulse rate, pulse width, open circuit voltage and peak current so long as the electrode remains the desired distance from the workpiece, resulting in no arcing, and no short circuit exists.
In the past, skilled EDM operators have used the sound of the sparking across the gap as a rough indication of proper sparking across the gap. This has been done by placing the ear of the operator close to the workpiece, or by placing the ear of the operator in sound contact with the workpiece through some sound conducting object, e.g., a wooden dowel placed in contact with the workpiece and the operator's ear.
This can be in some cases dangerous to the operator and is at least an inconvenience to the operator, having to place his ear near the workpiece or against the sound conducting object.